The present invention relates to a chip-type solid electrolytic capacitor and a method of manufacturing it.
Recently the number of chip components has been drastically increasing due to reduction of weight and dimensions of an electronic equipment and improvement in a surface packaging technique. While size reduction and capacity increase of a chip-type solid electrolytic capacitor are progressing, there has been an increasing demand for further size reduction of a chip component itself.
A conventional chip-type tantalum solid electrolytic capacitor has a structure shown in FIG. 10. More specifically, anodic oxidization is performed to form an oxidized dielectric film on the surface of a porous anode body including an anode lead wire 1 and made of tantalum which is a valve-metal. An electrolyte layer of manganese dioxide or the like is formed on this surface, and a carbon layer and a cathode layer 2 are laminated successively. Thus, a capacitor element 3 is constituted. The capacitor element 3 is coated with a resin shell 4 except for a projecting tip la of the anode lead wire 1 and an exposed portion 2a of the cathode layer 2. An anode metal layer 5 is coated and formed on that surface of the resin shell 4 where the anode lead wire 1 is led out, including the projecting tip 1a of the anode lead wire 1, and the peripheral surface adjacent to this surface. Also, a cathode metal layer 6 is coated and formed on a cathode-side end portion of the resin shell 4 including the exposed portion 2a of the cathode layer 2, and the peripheral surface adjacent to this surface.
Another chip-type tantalum solid electrolytic capacitor of this kind is disclosed in Japanese Patent Unexamined Publication No. 2-256222. More specifically, as shown in FIG. 11, a conductive material 7 is connected to form a convex portion on an end portion of a cathode layer 2 of a capacitor element 3, and a box-like resin shell 4 is transfer-molded with an anode lead wire 1 being led out on one end. After that, the conductive material 7 is exposed by cutting a certain amount of the resin shell 4 from the cathode-side end portion of the resin shell 4 in the opposite direction of the anode lead wire 1. Then, an anode metal layer 5 is coated and formed on that surface of the resin shell 4 where the anode lead wire 1 is led out, including a projecting tip la of the anode lead wire 1, and the peripheral surface adjacent to this surface, and also, a cathode metal layer 6 is coated and formed on a cathode-side end portion of the resin shell 4 including an exposed portion 7a of the conductive material 7, and the peripheral surface adjacent to this surface.
In the chip-type tantalum solid electrolytic capacitors shown in FIGS. 10 and 11, the resin shell 4 is molded with the anode lead wire 1 led out of the capacitor element 3 being maintained horizontally. If the anode lead wire 1 is bent and deformed due to hanging down of the capacitor element 3 caused by its own weight, contact trouble in the previous step before molding, or the like, the thin cathode layer 2 coated on that surface of the cathode layer 2 of the capacitor element 3 which is opposite to the surface where the anode lead wire 1 is led out, and the peripheral surface adjacent to this surface, will be brought into contact with a mold so that resin coating will be insufficient. As a result, many visual demerits are induced owing to exposures of the thin cathode layer 2. In order to prevent these drawbacks, there has been a demand for improving the accuracy when the capacitor element is inserted into the mold. Also, in the chip-type tantalum solid electrolytic capacitor shown in FIG. 10, when the cathode-side peripheral surface of the shell is exposed to the outside, part of the resin shell 4 is removed by sandblast. In this case, since the cathode layer 2 must be exposed while checking the condition of the exposure of the cathode layer 2 so as not to break it, this step is extremely difficult to carry out, thereby degrading the productivity. Further, because the cathode layer 2 is thin, a leakage current and a tan.delta. value of the capacitor will be increased unfavorably if the cathode layer 2 is broken.
Moreover, in the chip-type tantalum solid electrolytic capacitors shown in FIGS. 10 and 11, the contact strength of the anode lead wire 1 and the resin shell 4 with the anode metal layer 5 formed on the anode lead surface of the resin shell 4 is low, and the contact strength of the cathode metal layer 6 formed on the cathode lead surface of the resin shell 4 with the resin shell 4 and the conductive material 7 is low. In this case, as the outer shape of the chip-type tantalum solid electrolytic capacitor becomes smaller, the areas of formations of the anode metal layer 5 and the cathode metal layer 6 become smaller. Consequently, if bending or thermal stress of a printed board is caused after such a chip-type tantalum solid electrolytic capacitor is mounted on the printed board, there is especially induced a problem that the cathode metal layer 6 is peeled off, so that insulation deterioration is apt to occur. Also, if an oxide is attached to the surface of the anode lead wire 1, or if molded resin burrs remain there, there is a problem that the tan.delta. value is increased.
Furthermore, in the chip-type tantalum solid electrolytic capacitors shown in FIGS. 10 and 11, the moisture which has entered the capacitor from the outside is likely to degrade the properties of leakage current, tan.delta. and the like. Especially in order to reduce the size of the solid electrolytic capacitor and to increase its capacity, plating is conducted to form the anode metal layer 5 and the cathode metal layer 6 which constitute the terminal sides. In such a case, however, since the solid electrolytic capacitor is immersed in a plating solution, there is a problem that free water and ionic substances enter the capacitor element 3 and are apt to deteriorate electric properties.
Another chip-type tantalum solid electrolytic capacitor of this kind is disclosed, for example, in Japanese Patent Unexamined Publication No. 59-222919. More specifically, as shown in FIGS. 12A and 12B, an anode lead 10 is led out from a recessed section 9a of a resin shell 9 which coats a solid electrolytic capacitor element 8. The anode lead 10 is flattened thinly on a plane perpendicular to a direction of a notch which divides a top surface 9b of the resin shell 9 in two, and also, the anode lead 10 is bent at a right angle at a joint portion 10a and extended inside of the recessed section 9a to be led out along the recessed section 9a. After that, an anode-side conductive layer 12 and a cathode-side conductive layer 13 are formed on the periphery of the anode lead 10 led out from the recessed section 9a, the top surface 9b of the resin shell 9, and a cathode lead portion 11 exposed from the resin shell 9 by coating them successively with a single layer or a plurality of layers of each of a conductive adhesive, a conductive plate, a solder and so forth.
With the above-described structure of the chip-type tantalum solid electrolytic capacitor shown in FIGS. 12A and 12B, however, when the anode lead 10 is bent at a right angle at the joint portion 10a, there are induced some problems. For example, because the anode lead 10 is resilient, a mechanical stress is exerted on that portion of the anode lead 10 which is implanted in the resin shell 9, so that this stress will break the resin shell 9; the anode lead 10 is bent to have an unfavorably curved shape such that the anode lead 10 will extend out of the recessed section 9a of the resin shell 9; and the position of bending of the anode lead 10 is uncertain.